Simplify the following expression: $k = \dfrac{1}{10z} - \dfrac{8}{7z}$
Answer: In order to subtract expressions, they must have a common denominator. The smallest common denominator is the least common multiple of $10z$ and $7z$ $\lcm(10z, 7z) = 70z$ $ k = \dfrac{7}{7} \cdot \dfrac{1}{10z} - \dfrac{10}{10} \cdot \dfrac{8}{7z} $ $k = \dfrac{7}{70z} - \dfrac{80}{70z}$ $k = \dfrac{7 -80}{70z}$ $k = \dfrac{-73}{70z}$